Titania (TiO2) is stable physically and chemically and has a refractive index of greater than 2.5, which is greater than that of diamond having the greatest refractive among natural materials. If a refractive index is large in an optical material, the amount of light coming out of a medium having a low refractive index becomes large, and it is possible to reduce the size of the core in an optical waveguide and the thickness of an optical lens. Also, if highly refractive particles are dispersed in a polymer, the degree of whiteness will be increased by enhanced scattering of light. Titania is one of important industrial materials that have been used for white pigments for a long time owing to its high refraction characteristic. Further, it is one of the main components of piezoelectric materials, dielectrics, and semi-conductors according to the development of electronic industry since it is characterized by a high dielectric constant. Still further, it has been the trend recently that its application to cosmetics, photocatalytic thin layers, fillers, paints, lubricants, precision ceramics, etc. using UV shielding and absorbance properties as well as a catalyst for removing organic contaminants according to the chemical corrosion resistance and photocatalytic effect has been extended rapidly.
Titania (TiO2) exists in three main crystalline forms; anatase, rutile and brookite. It was well known that each phase has different physical properties such as refractive index, dielectric constant, and photochemical activity, etc. Among these three, brookite is neither a crystal type existing under general conditions nor important industrially. With respect to each use, the crystal structures of rutile and anatase, that are useful industrially, are reviewed below: Both of anatase and rutile are based on an octahedra (TiO62−) unit and have a tetragonal structure by occupying edges and corners in different ways, and are TiO2 stoichiometrically. However, in the rutile type, two encountering edges of each octahedra occupy each other and form linear chains in the (001) direction, where each chain is connected to each other by occupying oxygen atoms in corners. On the other hand, anatase has four occupied edges of each octahedra although it does not occupy corners. The anatase structure is shown to be zigzagged chains of octahedra and is connected to each other through the occupied edges thus occupying more edges than rutile does, but its interstitial space between octahedras is greater. Due to such structural difference, although rutile and anatase have the same chemical formula, the refractive index is about 2.7 for rutile while that of anatase is 2.5, and the dielectric constant of rutile is 114 and that of anatase is 31, meaning that of rutile is almost 4 times greater than that of anatase. Accordingly, for the uses related to optical or electronic materials, rutile is used more importantly for industry than anatase titania is.
Titania can be synthesized by several methods such as inert gas condensation, oxidation-hydrothermal synthesis of metallic Ti, frame hydrolysis of TiCl4, and sol-gel: There are many reports on the preparation of anatase particles with sizes raging from several nm to several microns and a variety of shapes. With regard to rutile titania powders, unlike anatase, it was known that the preparation of rutile particles, particularly nanosizses, is much more difficult. Thermodynamically stable rutile can be obtained by high temperature calcinations of the kinetically stable anatase phase. However, calcinations at a high temperature unavoidably led to the formation of large particle size and coagulation of nano particles making its use limited. In order to resolve such problems, many researchers have put their efforts into the manufacture of rutile titania nano particles at a low temperature. Reported in Korean Patent No. 2000-0066290; U.S. Pat. No. 6,440,383; R. R. Bacsa et al. J. Am. Ceram. Soc., 79, 2185 (1996); C-C Wang et al. Chem, Mater., 11, 3113 (1999); S. T. Aruna et al. J. Mater. Chem., 10, 2388 (2000); Y. Li et al. J. Mater. Chem., 12, 1387 (2002); S. J. Kim et al. J. Am. Ceram. Soc., 82, 927 (1999); and W. Wang et al. J. Phys. Chem. B. 108, 14789 (2004) are the methods of manufacture of rutile nano particles in an aqueous solution. However, in all these methods, rutile-phase titania nano particles are manufactured by hydrolysis methods at a room temperature by using TiCl4 and TiOCl2 as starting materials, or in the hydrothermal treatment method at 100-250° C. in an autoclave after re-dissolving titanium hydroxide or amorphous TiO2 obtained from TTIP, etc. by adding highly concentrated nitric acid or hydrochloric acid with citric acid. Undesirably, the rutile nano particles manufactured accordingly suffers from the presence of contaminant salts and, hence, the particles should be repeatedly washed to eliminate the salts. Such property makes rutile titania nano particles very limited in being used for electronic or optical materials.
Also, reported in JP-A 1987-283817 and Ichinose et al. J. Ceram. Soc. Jpn., 104, 715 (1996) are the methods of manufacture of titania nano sols from peroxotitanate solution, prepared by dissolving again titanium hydroxide with hydrogen peroxide. However, nano sol particles manufactured according to the above methods all have anatase crystals.
As described in the above, the conventionally manufactured rutile titania (TiO2) nano sols or particles are contaminated with a large amount of ionic impurities. Even if special processes for removing impurities are added, it is very difficult to remove impure ions adsorbed to the surface of nano particles completely in view of the characteristics of nano particles having a high specific surface area.
As a result of putting efforts into the resolution of the above-described problems, the inventors of the present invention invented a process for manufacturing nano-sized rutile titania sols having no ionic impurities at all by using only both high-purity TTIP and hydrogen peroxide.
Since the crystal phase of the rutile sols manufactured according to the present invention have a rutile structure completely, which means that it has a high refractive index and dielectric constant, with a particle size of less than about 30 nm, a very excellent dispersibility in an aqueous medium, and no ionic impurities at all, it may be used for optical materials having a high refractive index and transparent insulator films having a high dielectic constant for organic thin film transistors (OTFTs).